To protect an electrical equipment item, it is standard practice to use, between the two lines of an alternating mains supply, a metal oxide varistor, notably zinc oxide, mounted in series on one side with a thermofusible disconnection element and on the other side with a gas discharge arrestor.
Such a device works theoretically as follows: the gas discharge arrestor supports practically all the alternating voltage of the mains supply. Indeed, the stray capacitance of the gas discharge arrestor is of the order of a picofarad whereas the stray capacitance of the varistor is from a few to a few tens of nanofarads. When an overvoltage occurs, it causes the gas discharge arrestor to fire, which can be extinguished only if the so-called follow current, which subsequently passes through it, becomes sufficiently low. It is the resistance of the varistor which ensures the limiting of the follow current and allows the extinguishing of the gas discharge arrestor.
When an overvoltage protection device has operated a certain number of times or continuously following a prolonged overvoltage, its components come to the end of their life. For a gas discharge arrestor, the end of life corresponds to a short-circuit condition. On the other hand, for a varistor, the end of life can be reflected by an explosion for pulse phenomena or by a great reduction of its internal resistance (tending towards short-circuit) which can often go as far as its ignition. For safety, the gas discharge arrestor can be designed for its capacity to carry energy pulses linked to the overvoltages to be lower than that of the varistor. In this way, it is the gas discharge arrestor which comes first to its end of life and which is short-circuited.
The voltage of the mains supply is then totally diverted to the varistor which heats up and leads to the melting of the thermofusible element and thermal disconnection, that is to say that the protection device is taken out of service.
However, it is difficult to ensure the reliability of the disconnection produced by the melting of the thermofusible element.